I was under the impression (after a lot of research), that if you * as a personal user * use less than 2500Ltrs of BioFuel per Annum, then there is no need to register this with HMRC.
This amount being just about enough to cover the 12,500 miles, that an average person would cover in 12 months.

However, if you use over the 2500ltrs, then you do become liable for tax, and that applies to the whole amount, and not just any over the 2500ltrs.